EuroWire November 2021

Technical Article

Tension measurement faster than ever before: advanced stand-alone gauge head for quality control of optical fibres By Rebecca Zachau, corporate communications, Sikora AG, Germany

Introduction The fast, lossless transfer of data over long distances via optical fibre cables is essential for our increasingly digital and connected society. The future demand for optical fibre cables is further growing due to the global establishment and expansion of 5G wireless standards, increasing intra-data traffic within data centres as well as data traffic between different data centres. Visions of autonomous driving, vital functions monitoring IoT medical devices or smart cities with automated trafficmanagement are only possible by establishing an infrastructure with widespread optical fibre networks. They enable an extreme high bandwidth and a constantly increasing intra-data traffic – permanent and in real-time. In order to ensure an optimal quality of the optical fibre, a variety of measuring and control technologies are already applied in the drawing process at different production stages. Tension has an important impact on the optical fibre quality, which influences the uncoated fibre. Sikora AG has developed a stand-alone gauge head with a measuring rate of up to 10 kHz that measures the tension quickly and precisely. Therefore, it is particularly intended for optical fibres that are further processed to premium optical fibre cables and for which specific requirements and standards exist. A gauge head with such a high measuring rate is especially suitable for fast control circuits. Requirements for the manufacturing of optical fibres and optical fibre cables In the course of rapidly growing worldwide networking, require- ments on single-mode as well as multi-mode optical fibres with regard to their relevant parameters are ever increasing.

Consequently, the tolerances for attenuation, loss and cut-off frequency aswell as geometrical dimensions are continually getting tighter. Optical fibre manufacturers have to take the corresponding optical fibre specifications for the target application into account. Only then, they receive exact products that effectively fulfil the guaranteed durability. Therefore, continuous online quality control during the entire drawing process is crucial. During the different production stages, gauge heads and processor systems measure, monitor and control the entire drawing process to ensure the quality of the optical fibre and to increase the efficiency of the production process. The objective of optical fibre manufacturers is to control as many variables as possible in the drawing process to produce a fibre with a grade of high specifications as well as minimum scrap production. For example, the diameter of the fibre is controlled over the line speed. When controlling one parameter, the influenced factors must also be monitored because all parameters in the drawing process are linked together. Hence, as the preform temperature increases, the viscosity of the optical fibre decreases. When line speed and tension stay the same, consequently, the diameter decreases and scrap is produced or the fibre can even break. Therefore, for higher production line speeds at consistent tension, the preform temperature is increased. On one hand, the tension must be controlled to prevent fibre breakage. On the other hand, the controlled tension guarantees that important parameters of the produced fibre, such as the cut- off frequency of the attenuation parameter, comply with the set specifications. Innovative tension measurement based on the birefringence principle There are different methods for tension measurement available on the market. One of these is the traditional vibration measuring method where a gauge head is used for the measurement of the diameter as well as the vibration of the optical fibre in the drawing process and thus determines the tension. There is no distinction between natural and forced vibration of the fibre, whereby measuring inaccuracies may occur due to stronger external environmental influences. The application of such a dual measuring system is beneficial, for example, when a lack of space in the drawing tower makes the installation of multiple stand-alone gauge heads difficult. Sikora offers a new technology for a precise tension measurement in a stand-alone system. This is based on the birefringence principle, which uses the direction-dependent refractive characteristics of glass that occur when forces act on the glass.

▲ Figure 1 : Measuring principle based on the birefringence principle: the tension is determined by the change in polarisation of the light entering the optical fibre relative to the double refracted emitting light that hits the receiving unit

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November 2021